In certain video disc systems, video information is recorded by means of geometric variations in the bottom of a smooth spiral groove on the surface of a disc record. The disc record surface includes a coating of conductive material which is preferably covered with a thin deposit of dielectirc material. A signal pickup subject to engagement with the spiral groove includes a conductive surface which cooperates with the conductive coating and the dielectric deposit of the disc record, to form a capacitor. When the disc record is rotated, an edge of the conductive surface of the signal pickup, while riding in the disc record groove, recovers capacitive variations due to geometric variations in the bottom of the smooth spiral groove. The capacitive variations representing prerecorded video information are processed and applied to a conventional television receiver for reproduction. The variable capacitor concept is described in detail in the U.S. Pat. No. 3,842,194, issued to J. K. Clemens, entitled "INFORMATION RRECORDS AND RECORDING/PLAYBACK SYSTEMS THEREFOR", and assigned to the present assignee.
The capacitive variations between the conductive coating of the disc record and the conductive surface of the signal pickup are a part of a tuned circuit, wherein the resonant frequency of the tuned circuit varies as the capacitance between the signal pickup and the disc record changes. A flexibly mounted conductive member (e.g., pickup arm) is provided, with its free end electrically and mechanically coupled to the conductive surface of the signal pickup, and the pivoted end movably connected to a hollow box-lixe support housing, also made of conductive material. The conductive member mechanically supports the signal pickup, and also forms a transmission line with the surrounding conductive support housing. The tuned circuit includes the reactance of the transmission line and the capacitance between the signal pickup and the disc record. The tuned circuit is energized by a fixed, high frequency (for example, 915 MHz) oscillator. The response of the tuned circuit to the oscillator excitation signal changes as a function of the prerecorded information on the disc record. The output signals from the tuned circuit, indicative of information prerecorded on the disc record, are subsequently applied to a suitable signal processing circuit coupled to a television receiver for image display. The transmission line concept is described in detail in the copending U.S. Pat. No. 3,872,265, issued to S. E. Hilliker, entitled "VIDEO DISC TRANSMISSION LINE AND STYLUS RF RETURN SYSTEMS", and assigned to the present assignee.
In order to prerecord a reasonable length of programs, video disc systems of the aforementioned type employ disc records having a relatively high groove density (e.g., 4,000 to 6,000 grooves per inch). The use of high groove densities results in fine groove dimensions. For example, the groove separation may be 3.5 microns and the groove depth may be 2.5 microns. It has been found that the fragile walls of the video disc record grooves cannot be dependably relied upon for tracking (that is, driving the signal pickup support housing during playback radially inward across the prerecorded surface of the disc record). Therefore, the support housing, carrying the signal pickup, is driven radially inward by a suitable drive mechanism in proper time relationship with the rotational speed of the disc record. The drive mechanism drives the signal pickup radially inward toward the center of rotation of the disc record such that the attitude of the signal pickup conductive surface is held relatively constant with respect to the groove.
In an above-mentioned type system, returning of the support housing after a playback cycle to a rest position (so that the playback sequence may be later repeated) becomes quite complicated, especially where the support housing rest position is above the starting groove in the disc record. The drive motor, employed for radially inward movement of the support housing, may be reversed at the end of a playback cycle causing the support housing to travel radially outward to the rest position. Such an arrangement may be undesirable. First, a motor with a reversible feature means additional cost. Second, the return of the support housing may take as much time as a real time playback (e.g., 15 to 20 minutes for a long-playing disc record), unless a rather expensive two-speed reversible drive motor is provided. That may mean that, between any two successive playbacks, the viewer would wait for a relatively long interval of time to allow the support housing to return to the rest position. An alternative may be employment of a separate high speed motor for a rapid return of the support housing to the rest position. The alternative of providing a separate high speed motor may be disadvantageous for reasons such as cost and complicated linkages connecting the separate motor to the drive mechanism. One of the objects of the present invention is to provide an improved signal pickup return mechanism which is simple and rugged in construction easy to install, inexpensive to manufacture, and yet efficient and reliable in operation.
It may be beneficial to locate the support housing, carrying the signal pickup, and the drive mechanism in the cover of the player. For example, an advantage may be that the pickup support housing need not clear the disc record between successive playback cycles in a manual player; because when the cover is lifted away from the base, the support housing and the drive mechanism would be automatically cleared out of the way. Another advantage may be that the 915 MHz transmission line in the Hilliker type playback system (U.S. Pat. No. 3,872,265) could be enclosed in the cover in order to provide an additional shielding for preventing deleterious effects on the surrounding electronic equipment. It is another object of the present invention to provide a signal pickup return mechanism that is compatible with an arrangement whereat the support housing and the drive mechanism are located in the cover of the player.